(i) Field of the Invention
The present invention relates to a plasma torch intended for the excitation of a gas for the purpose of analyzing it.
The invention also relates to a gas analyzer using such a plasma torch.
(ii) Description of Related Art
Currently, gas analysis techniques are indirect techniques, such as filtration, hydrolysis or sparging, in which the impurities, the concentration of which is to be determined, are extracted from the gas before analysis.
Thus, for example, the filtration analysis technique uses a membrane for filtering the gas to be analyzed for the purpose of retaining the impurities that it contains. Next, these impurities are dissolved in an acid solution and then analyzed, for example by spectroscopy, for the purpose of determining the nature and concentration thereof.
These conventional analytical techniques have a number of drawbacks.
First of all, because of their nature and particularly because there is a step of extracting the particles to be analyzed, these techniques are not suitable for the quality of a gas to be analyzed continuously.
Furthermore, they give relatively inaccurate results since these techniques only allow an average concentration value corresponding to the total amount of the sample to be obtained. They therefore do not allow instantaneous variations in the concentrations to be detected.
In addition, some impurity particles are likely to be in the form of volatile compounds which cannot be extracted from the gas using such techniques. The result obtained is thus likely to be underestimated.
Finally, these techniques entail a not insignificant risk of contaminating the gas and require relatively complex equipment.
It has been attempted to remedy these drawbacks by using a direct gas analysis technique.
According to this technique, a gas sample to be analyzed is introduced into a heat source, such as a plasma, capable of dissociating the chemical species present in the sample into free atoms and then of exciting and optionally ionizing the atoms obtained. Next, these excited atoms are detected by measuring the various wavelengths that they emit or, if they are ionized, by measuring their mass.
Although this technique allows a gas to be analyzed continuously it also has a number of drawbacks, especially because of the gas recirculation movements produced by the action of Lorentz forces near the inductor used for generating the plasma.
These recirculation movements will force the gas to the periphery of the plasma and cause the decomposition products to be deposited on the torch and therefore cause undesirable contamination of the latter, impeding optical detection, as well as modifying the energy transfer between the induction coil and the plasma.
Moreover, the gas flowing in this peripheral region undergoes less excitation, thereby helping to reduce the accuracy of the measurement.
The studies completed by the Applicant on this subject have moreover demonstrated that, depending on the nature of the gas to be analyzed (for example, depending on whether or not it is a diatomic gas), there is a major risk of the plasma being blown out when introducing the gas to be analyzed into this plasma.